An inflatable vehicle occupant protection device, such as an air bag, is inflated upon the occurrence of a vehicle crash. The air bag is part of an apparatus which further includes a crash sensor and an inflator. The crash sensor senses vehicle conditions that indicate the occurrence of a crash. When the crash sensor senses a crash-indicating condition of at least a predetermined threshold level, the inflator is actuated and emits inflation fluid which inflates the air bag into the vehicle occupant compartment. The air bag can then engage a vehicle occupant to help protect the occupant from a forceful impact with parts of the vehicle as a result of the crash.
The manner in which the inflating air bag engages the vehicle occupant can be influenced by conditions such as the force with which the occupant moves against the air bag and the pressure of the inflation fluid in the air bag. Those conditions, in turn, can be influenced by vehicle conditions, such as the severity of the crash, and/or by vehicle occupant conditions, such as the size, weight and position of the occupant. Therefore, it may be desirable to regulate the inflation fluid pressure in the air bag in response to one or more of these conditions.